VCO

So I think it’s time somebody posted to the physics blog…where has everyone been all July?

I am currently 6 weeks into my REU, with 4 more to go. Up to now I’ve been working on assembling and testing a voltage controlled oscillator (VCO). The VCO is part of the laser frequency stabilization control loop here at LIGO. The interferometer requires a very stable laser beam because fluctuations in the laser’s frequency that is input to the interferometer are indistinguishable from a change in the lengths of the interferometer arms (the thing we’re trying to measure). The laser that we currently use is far too noisy out of the box. (Laser noise by-the-way comes from things like electrical jitter in power supplies and control circuits, thermal noise acting on the laser cavity, and the optical pumping that makes the laser lase). To correct for noise, we take the beam before it is injected into the main interferometer and pass it through several (active and passive) stages that eliminate excess noise. The VCO device that I am working on is a single electronic (rack mount format) component that is part of the stage that eliminates high frequency noise (in a range of ~10kHz to 100kHz). The VCO takes as input a DC error signal that tells how far the laser is from a target/reference frequency. It turns this DC signal into an AC (RF) signal who’s frequency is modulated proportional to the DC voltage. The AC signal is used to drive an acousto-optic modulator (AOM) that shifts the frequency of the laser’s light by the frequency that it is driven with, thereby “locking” the laser to the reference frequency.

We anticipate that we will need to modulate the laser’s frequency by plus-minus 1MHz. What makes this complicated is that any VCO that can be purchased commercially (they’re essentially diodes that have a voltage dependent capacitance) has way too much intrinsic frequency noise–we’d end up introducing a second noise source into the laser rather than eliminating existing noise. To get around this, we start with a VCO that operates at much higher frequencies than we need (this typically means a higher signal-to-noise ratio) and divide the signal down in frequency, thereby dividing down the noise by the same amount. In our setup we start with a 1GHz signal that we can modulate plus-minus 130MHz and divide it down to an 8 plus-minus 1MHz signal (our target frequency gain). Since the AOM needs ~80MHz to operate, we then mix this signal with an extremely stable reference signal from a crystal oscillator to obtain an 80 plus-minus 1MHz signal with low frequency noise–at least in theory!

The VCO is in the prototype stage so I’ve spent the last several weeks working out bugs in the design. It was initially laid out entirely on paper (CAD in reality) and specifications for custom parts were sent off to manufacturers. We’ve had several issues with the physical layout of components (you can’t fit two parts in the same space in real life) and I’ve spent some time correcting resistor values on this chain of op-amps. All things considered, I’m quite impressed that my advisor made so few design errors, given the complexity of the device.

I have also been testing for, characterizing and eliminating noise sources in the circuitry. I’ve replaced a bunch of noisy (probably blown) op-amps and have consequently become fairly good at delicate soldering. (I replaced a ridiculously small chip the other day…about 1/4 inch square, with 10 pins on two sides.)

As of now, I’m waiting on some parts that have yet to arrive before I can do any more meaningful tests. I have a project report due next week, so it’s actually nice to have a little downtime right about now. Once the parts arrive, I’ll be able to start assembling a second VCO (LIGO has two interferometers after all) and I’ll be able to do more extensive noise tests and in addition to side-by-side comparisons between the two identical (hopefully) boxes.

I have to admit that eastern Washington is not quite as fun as it sounds like Germany is, but the local farmer’s market is quite nice and we’re a short drive from the mountains (where I spent a weekend camping out, see picture). I’m thinking I’ll spend a weekend at Hells Canyon sometime in the future when it’s not too hot.

The VCO:

Northeast Oregon (panorama shot):

keine titel.

Hello people who are interested in Germany/Joey/Physics!!

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!im sorry, but my posts are always super long…

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So a bunch of exciting stuff has happened since the last post. So I will start Friday June 19th, about 2 weeks ago… I had just received my train passes from DAAD (the people who are paying me) which will allow me 5 free days of travel within Germany. I was thinking that I only had a few weekends left and thus I needed to use 2 of those passes and go somewhere awesome. Thus, around midnight Friday, I decided I wanted to go to Munich (Munchen in Germany) on Saturday. In the morning I got up, packed, booked a hostel, put on my running shoes and ran to make an 8am train. I must say, I made pretty good time from my room to the train station, and the train literally left less than 40 seconds after I got on it. My legs were still shaking when I had to change trains in Frankfurt about 40 min later!

Eventually I made it to Munich, checked into my hostel, and went about trying to find stuff I’d heard about that was cool. After a while of being lost underground (subway tunnels and such) I was able to make it to the famous Nymphenburg Palace with the help of some super-nice german people. It was beautiful and amazing and it had a garden which was over 500 acres. It was so strange to be walking down a busy street in Munich and then suddenly be in front of an astounding sight like this! I spent a little bit of time walking/being semi-lost in the garden and looked around the palace before moving on. Soon after, I was lucky enough to see a group of street performers composed of a cello, a stand up bass, a flute, and a mini-grand piano!! It was really cool!

From there I saw some statues and other landmarks, before stumbling across the Residence, which was the home of the rules of Bavaria for hundreds of years. I did not mean to find this (i was looking to find a theater and see a play) but it was amazing. I never went inside, but instead just admired the myriad statues and the great architecture outside. I walked around it for hours, passing gardens, fountains, and other incredible sights. There was a moment when I was in the middle of a garden in a small, ordinate pavilion while a street performer was playing solo violin and a man was painting a picture of him. The sun was setting behind a palace and it was all breathtaking.

Nothing else spectacular happened that day, and I again spontaneously decided to take a train in the morning. This time the train was at 7am and went to Garmisch-Partenkircken, a small town essentially in the Alps. I spent a really long time walking around the town and surrounding area. I eventually ended up at the top of the tallest mountain in Germany (with a little help from a train)!! It was almost 10,000 ft up, but I couldnt see much of the famed panoramic view of the Alps due to clouds. It was actually very disappointing… but I did walk on a glacier for a while! I finished off the weekend with a bit of shopping and I got home around 2am. Overall it was a great weekend and I really liked exploring so much stuff on my own!

Okay, so then I was back at work for a week. Early in the week I finished another program, which I talked about in my last blog post. It uses a first order partial differential equation to model neutron stars. When I was done with the program and had checked everything, it was still giving dramatically wrong results. I spent the entire remainder of the week checking units and algorithms in this program. We wrote up a mathematica script which could do some of the same things and our program agreed exactly. I checked several places within the program by computing values in multiple ways and comparing them. After a while my PhD student also started looking at it and determined that my program was probably right, but we were just missing some equation that is supposed to be “understood”! We still haven’t figured it out…

During this week the final few RISE students (RISE works for DAAD, and together they give us money) got here, so now there are 10 of us. There are 8 biology people and 2 physics. There is another physics person!! She is from England and she is hardcore experimental… so she didn’t really know anything about my project at all (because she is experimental… not because she is from England!). She mentioned that she wants to learn fortran, and I had to hold my tongue to not tell her that it is the devil, because no matter how much I dislike it, it is the standard in a lot of physics right now and if she is only going to know one language that should be it.

It was, overall, a pretty uneventful and frustrating week at work… so now I will skip forward to the weekend! I wasn’t sure what I was going to do until about 11:30 pm on Friday (I sense a pattern…) but then I decided to accompany some of the other RISE students to Hamburg. It was an 8 hour train ride, because we opted for the super cheap tickets (with a lot of train changing) and because we missed one train (which only added 1 hour). I had ordered some burger king and the lady was taking a while, I said we should abort but the other students thought we had time…

Anyway, Hamburg is a really cool city! It is a port town, and the people equally like fish and pirates. We got a hotel which I thought was really cool, because it was so small and had cool pictures everywhere, but the girls thought was a little sketchy, for the same reasons. Apparently the red light district in hamburg is some sort of historical big deal, and a thing called a “whore tour” is offered, where a woman dressed like a whore from hundreds of years ago shows you famous old brothels and tells interesting facts. I think this would have been hilarious/awesome/uncomfortable/memorable, but it was too expensive. So we toured some boats, some churches, a WWII memorial, and miniature wonderland and we also climbed a tall tower. Miniature Wonderland is like a model train set times 10^7! It was really awesome and through random confusion and awkwardness I was able to look at it for about an hour for free! Its kinda a long story…

So there is a flea market known as “the fish market” at 5am, and thus we decided to stay up until then! We went out to dinner at an awesome Italian place where I ordered coffee and everyone else ordered water, but the guy made fun of me and brought everyone beer! The two girls who went were really into going to clubs, so them and the other boy who went and myself ended up at a dance club all night. Its not really my scene, but it wasn’t Casey’s (the boy) scene either so it worked out. The name of the club was HALO, which was awesome. Oh, and we passed “the doll house”, which was the nightclub recommended to us by the ~40 year old woman at the travel information center, and it turned out to be a strip club!! From there my subpar german got us to the fish market, but everyone else was too tired and lame to stay up so we went to the hotel and slept for 3 hours.

The next day we ate free breakfast, checked out the motorcycle convention going on at the time, and went to the awesome hamburg zoo. I fed an elephant and a giraffe!! Also, in a lot of places the animals (ie the elephants, mountain goats, giant rabbit creatures, …) were not separated from the people at all! It was really unsafe/cool! It was fun and there was also a large guinea pig village with a church, windmill, barn, etc…

From there we ran to make our train. We passed a taxi that I wanted to hail, but the other students told me not to. Oh how I regret that moment… We missed the train and would have made it if I had said “screw you guys, hey taxi!” It would have been awesome… well, we just had to wait 1 hour for the next train, and then we came back home to Giessen!

Oh, and on the trip I finished the 5^th book ive read since ive been here! It is nice to have some time to read… not like at school where I have to do homework all the time! Next I’ll be reading GEB, and I’m really excited! Jeez this post is getting long…

So then this week started! We have given up on fixing the code from last week, and my PhD student said he trusts it and will figure out what we are missing sometime. So I started a new project, and he said “I know there is not much time left, so I don’t know if you will have a chance to finish it, which is fine.” To me that means “I CHALLENGE YOU TO FINISH THIS!” Thus, I am now in a race to create code that will find phase transitions in nuclear matter with 8 particles. It sure would have been cool if I had taken critical phenomena last semester… The code is going alright so far and I have worked on it a bit at home since I was challenged to complete it. I’m getting okay shapes for the curves, but the magnitude is off by a lot. It is probably a unit thing and I feel pretty confident that I will have it all worked out in a few days.

Tomorrow is the annual theoretical physics summer excursion! This year the excursion consists of riding bicycles between beer gardens! Yay! Then yesterday I learned that we are riding almost 50 miles! I haven’t ridden a bicycle more than from Tom & Ian’s to campus (not even 1/2 mile) since middle school… oh no! Eh, I’m sure I’ll be fine… but maybe all the beer gardens won’t help!

You’ve almost made it to the end of the post… just hold on a little longer!

So here is what I’ve got left ahead of me:

-bike trip tomorrow

-weekend in Berlin with my PhD student

-3 days of work

-4 day RISE conference and hanging out in Heidelberg (beautiful german city)

-1 day of work

-1 day to pack/check out of my room

-come home on Wed, July 15^th!!!

Wow I only have 4 days of work left! It’s going to be a challenge to get this program done! Anyway, you’ve made it to the end of this blog post! I’m sorry it was so long…

Have fun!

joey